U.S. patent number 7,794,553 [Application Number 11/952,694] was granted by the patent office on 2010-09-14 for thermoplastically processable amorphous metals and methods for processing same.
This patent grant is currently assigned to California Institute of Technology. Invention is credited to Gang Duan, William L. Johnson, Aaron Wiest.
United States Patent |
7,794,553 |
Duan , et al. |
September 14, 2010 |
**Please see images for:
( Certificate of Correction ) ** |
Thermoplastically processable amorphous metals and methods for
processing same
Abstract
High strength, thermoplastically processable (TPF) amorphous
alloys composed of Beryllium and at least one ETM and at least one
LTM, as well as methods of processing such alloys are provided. The
TPF alloys of the invention demonstrate good glass forming ability,
low viscosity in the supercooled liquid region (SCLR), a low
processing temperature, and a long processing time at that
temperature before crystallization.
Inventors: |
Duan; Gang (Chandler, AZ),
Johnson; William L. (Pasadena, CA), Wiest; Aaron (Los
Angeles, CA) |
Assignee: |
California Institute of
Technology (Pasadena, CA)
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Family
ID: |
39496569 |
Appl.
No.: |
11/952,694 |
Filed: |
December 7, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080135138 A1 |
Jun 12, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60873515 |
Dec 7, 2006 |
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60881960 |
Jan 23, 2007 |
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60923221 |
Apr 13, 2007 |
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Current U.S.
Class: |
148/403;
148/421 |
Current CPC
Class: |
C22C
45/00 (20130101); C22F 1/00 (20130101) |
Current International
Class: |
C22C
45/10 (20060101) |
Field of
Search: |
;148/403 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
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other.
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Primary Examiner: Wyszomierski; George
Attorney, Agent or Firm: Kauth, Pomeroy, Peck & Bailey
LLP
Government Interests
STATEMENT OF FEDERAL RIGHTS
The U.S. Government has certain rights in this invention pursuant
to Grant No. DMR0520565 awarded by the National Science Foundation.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The current invention claims priority to U.S. Provisional
Application No. 60/873,515, filed Dec. 7, 2006, U.S. Provisional
Application No. 60/881,960, filed Jan. 23, 2007, and U.S.
Provisional Application No. 60/923,221, filed Apr. 13, 2007, the
disclosures of each of which are incorporated herein by reference.
Claims
What is claimed is:
1. A thermoplastically processable bulk solidifying amorphous alloy
having a composition in accordance with the equation:
(Zr.sub.xTi.sub.(1-x)).sub.a1ETM.sub.a2Cu.sub.b1LTM.sub.b2Be.sub.c,
where (ETM) comprises the group of Early Transition Metals, (LTM)
comprises the group of Late Transition Metals; where x is an atomic
fraction and a1, a2, b1, b2, and c are atomic percentages, and
where (a1+a2) falls within the range of 60 to 80%, x is in the
range of 0.05 to 0.95, (b1+b2) is in the range of 2 to 17.5%, c is
at least 15%, and Ni comprises no greater than 5% of the overall
composition; and where the alloy has a supercooled liquid region
(.DELTA.T) defined as the temperature difference between the glass
transition temperature and crystallization temperature of the alloy
of at least 135 K and a viscosity within this supercooled liquid
region that falls below a value of less than about 10.sup.5 Pa-s
when measured at a heating rate of 20 K/min.
2. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the alloy has a composition in accordance
with the following equation: Zr.sub.aTi.sub.bCu.sub.cBe.sub.d; and
wherein a, b, c, and d are atomic percentages, a+b is within the
range of 60 to 80%, and d is greater than or equal to 15%.
3. The thermoplastically processable bulk solidifying amorphous
alloy of claim 2, wherein a is approximately equal to five times b
and d is greater than or equal to 20%.
4. The thermoplastically processable bulk solidifying amorphous
alloy of claim 2, wherein a is approximately equal to b.
5. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the atomic percent of Zr and Ti is in the
range of from about 60 to 75%.
6. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, further comprising up to 5% of at least one
additional material.
7. The thermoplastically processable bulk solidifying amorphous
alloy of claim 6, wherein the additional material is selected from
the group consisting of tin, boron, silicon, aluminum, indium,
germanium, gallium, lead, bismuth, arsenic and phosphorous.
8. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, further comprising up to 15% of at least one
additional early transition metal.
9. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the early transition metal is selected
from the group consisting of chromium, hafnium, vanadium, niobium,
yttrium, neodymium, gadolinium and other rare earth elements,
molybdenum, tantalum, and tungsten.
10. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, further comprising up to 15% of at least one
additional late transition metal.
11. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the early transition metal is selected
from the group consisting of manganese, iron, cobalt, ruthenium,
rhodium, palladium, silver, gold, and platinum.
12. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the alloy has an amorphous phase that
comprises greater than 25% of the alloy by volume.
13. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the alloy has an amorphous phase that
comprises greater than 90% of the alloy by volume.
14. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the alloy has a density of around 5.5
g/cm.sup.3.
15. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the alloy has a supercooled liquid region
of greater 140 K when measured at a heating rate of 20 K/min.
16. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein at a heating rate of 20 K/min the alloy
attains a viscosity in the supercooled liquid region of lower than
10.sup.4 Pa-s.
17. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the alloy has a critical cooling rate of
less than 10.sup.6 K/s.
18. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the alloy has a critical cooling rate of
less than 10.sup.3 K/s.
19. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the composition has a .DELTA.T of at
least 160 K and is selected from the group consisting of
Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5,
Zr.sub.37.5Ti.sub.25Cu.sub.10Be.sub.27.5, and
Zr.sub.40Ti.sub.25Cu.sub.10Be.sub.25.
20. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the alloy has a critical casting
thickness of greater than 1 mm.
21. The thermoplastically processable bulk solidifying amorphous
alloy of claim 1, wherein the alloy has a critical casting
thickness of greater than 15 mm.
22. A thermoplastically processable bulk solidifying amorphous
alloy having a composition in accordance with the equation:
(Zr.sub.xTi.sub.(1-x)).sub.a1ETM.sub.a2Cu.sub.b1LTM.sub.b2Be.sub.c,
where (ETM) comprises the group of Early Transition Metals, (LTM)
comprises the group of Late Transition Metals; where x is an atomic
fraction and a1, a2, b1, b2, and c are atomic percentages, and
where (a1+a2) falls within the range of 60 to 75%, x is in the
range of 0.50 to 0.85, (b1+b2) is in the range of 2 to 17.5%, c is
in the range of 17.5 to 33%, and Ni comprises no greater than 5% of
the overall composition; and where the alloy has a supercooled
liquid region (.DELTA.T) defined as the temperature difference
between the glass transition temperature and crystallization
temperature of the alloy of at least 135 K and at a heating rate of
20 K/min has a viscosity within this supercooled liquid region that
falls below a value of less than about 10.sup.5 Pa-s.
23. The thermoplastically processable bulk solidifying amorphous
alloy of claim 22, wherein the alloy has a composition in
accordance with the following equation:
Zr.sub.aTi.sub.bCu.sub.cBe.sub.d; and wherein a, b, c, and d are
atomic percentages, a+b is within the range of 60 to 65%, c is in
the range of 5 to 17.5%, and d is in the range of 17.5 to 32%.
24. The thermoplastically processable bulk solidifying amorphous
alloy of claim 23, wherein a is approximately equal to five times b
and d has a lower limit of 20%.
25. The thermoplastically processable bulk solidifying amorphous
alloy of claim 22, further comprising up to 5% of at least one
additional material.
26. The thermoplastically processable bulk solidifying amorphous
alloy of claim 25, wherein the additional material is selected from
the group consisting of tin, boron, silicon, aluminum, indium,
germanium, gallium, lead, bismuth, arsenic and phosphorous.
27. The thermoplastically processable bulk solidifying amorphous
alloy of claim 22, wherein the early transition metal is selected
from the group consisting of chromium, hafnium, vanadium, niobium,
yttrium, neodymium, gadolinium and other rare earth elements,
molybdenum, tantalum, and tungsten.
28. The thermoplastically processable bulk solidifying amorphous
alloy of claim 22, wherein the late transition metal is selected
from the group consisting of manganese, iron, cobalt, ruthenium,
rhodium, palladium, silver, gold, and platinum.
29. The thermoplastically processable bulk solidifying amorphous
alloy of claim 22, wherein the composition has a .DELTA.T of at
least 160 K and is selected from the group consisting of
Zr.sub.37.5Ti.sub.25Cu.sub.10Be.sub.27.5,
Zr.sub.40Ti.sub.25Cu.sub.10Be.sub.25, and Zr.sub.35Ti.sub.30
Cu.sub.7.5Be.sub.27.5.
Description
FIELD OF THE INVENTION
The current invention is directed to high strength amorphous alloys
that can be thermoplastically processed to make material parts and
articles, and methods of thermoplastically processing such
amorphous alloys.
BACKGROUND OF THE INVENTION
Over the last two decades metallic glasses (MGs) have received
increasing attention because of their unique characteristics, such
as high strength, high specific strength, large elastic strain
limit, excellent wear and corrosion resistance, along with other
remarkable engineering properties. (For further discussion see,
e.g., A. L. Greer, Science 1995, 267, 1947; W. L. Johnson, MRS
Bulletin 1999, 24, 42; A. Inoue, Acta Materialia 2000, 48, 279; D.
H. Xu, G. Duan, and W. L. Johnson, Physical Review Letters 2004,
92, 245504; V. Ponnambalam, et al., Journal of Materials Research
2004, 19, 1320; and Z. P. Lu, C. T. Liu, J. R. Thompson, W. D.
Porter, Physical Review Letters 2004, 92, 245503, the disclosures
of which are incorporated herein by reference.) Because of the
promise shown by these materials, researchers have designed a
multitude of multi-component systems that form amorphous glassy
alloys, among which Zr-- (U.S. Pat. No. 5,288,344, referred to as
Vit1 series of alloys, the disclosure of which is incorporate
herein by reference) bulk metallic glasses (BMGs) have been
utilized commercially to produce a variety of items, including, for
example, sporting goods, electronic casings, and medical
devices.
Most practical applications of MGs demand near-net-shaping process
in manufacturing. However, conventional die casting, the common
technique for net-shape processing of metals, requires fast cooling
to bypass the crystallization of most MGs during solidification.
This fast cooling requirement limits the ability to make pieces of
large cross-section (i.e., limited by critical casting thickness),
limits the ability to make parts with high aspect ratios (i.e.,
with large thin walls), and limits the ability to make high quality
casts or to manufacture structures with complex geometries.
Nevertheless, the properties of these MGs, including their high
glass forming ability, good processability, large supercooled
liquid region (SCLR), and a viscosity that varies continuously and
predictably in the supercooled liquid region continues to hold out
the promise that they could be processed thermoplastically if
suitable candidate materials can be identified.
The unique advantages of injection molding, blow molding,
micro-replication, and other thermoplastic technologies are largely
responsible for the widespread uses of plastics such as
polyethylene, polyurethane, PVC, etc., in a broad range of
engineering applications. Powder Injection Molding (PIM) of metals
represents an effort to apply similar processing to metals, but
requires blending of the powder with a plastic binder to achieve
net shape forming and subsequent sintering of the powder. Given
suitable materials, thermoplastic forming (TPF) would be the method
of choice for manufacturing of metallic glass components because
TPF decouples the forming and cooling steps by processing glassy
material at temperatures above the glass transition temperature
(T.sub.g) and below the crystallization temperature (T.sub.x)
followed by cooling to ambient temperature. (See, e.g., J.
Schroers, JOM 2005, 57, 35; and J. Schroers, N. Paton, Advanced
Materials & Processes 2006, 164, 61, the disclosures of which
are incorporated herein by reference.)
Thermoplastic forming (TPF) of MGs is a net-shaping processing
method taking place in the supercooled liquid region of such
materials, which is the temperature region in which the amorphous
material first relaxes into a viscous metastable liquid before
crystallization. Operating in this supercooled liquid region, TPF
decouples the fast cooling and forming of MG parts and allows for
the replication of small features and thin sections of metals with
high aspect ratios. TPF has several advantages over conventional
die casting, including smaller solidification shrinkage, less
porosity of the final product, more flexibility on possible product
sizes, a robust process that does not sacrifice the mechanical
properties of the material, and no cooling rate constraints on the
thickness of parts that can be rendered amorphous (critical casting
thickness).
From a processing point of view, MG alloys with an extremely large
supercooled liquid region (excellent thermal stability against
crystallization), which can provide lower processing viscosities
and exhibit smaller flow stress, would be desirable for use in
conjunction with a TPF process. In addition, excellent glass
forming ability and low glass transition temperature (T.sub.g) are
also preferred to thermoplastically process MGs. Unfortunately,
among the published metallic glasses, only the expensive Pt-, and
Pd-based glasses have shown good thermoplastic formability. (See,
e.g., J. Schroers, W. L. Johnson, Applied Physics Letters 2004, 84,
3666; G. J. Fan, et al., Applied Physics Letters 2004, 84, 487; and
J. P. Chu, et al., Applied Physics Letters 2007, 90, 034101, the
disclosures of which are incorporated herein by reference.)
Zr-based metallic glasses, especially the Vitreloy series, are much
less expensive than Pt- and Pd-based alloys, have exceptional glass
forming ability, but they are usually strong liquids (the drop of
viscosity with temperature is not steep) and low processing
viscosities are unattainable in the supercooled liquid region
(SCLR) between T.sub.g and T.sub.x. (See, e.g., A. Masuhr, et al.,
Physical Review Letters 1999, 82, 2290; R. Busch, W. L. Johnson,
Applied Physics Letters 1998, 72, 2695; F. Spaepen, Acta
Metallurgica 1977, 25, 407; and J. Lu, G. Ravichandran, W. L.
Johnson, Acta Materialia 2003, 51, 3429, the disclosures of which
are incorporated herein by reference.) One exception to this
general rule is Vit1b
(Zr.sub.44Ti.sub.11Cu.sub.10Ni.sub.10Be.sub.25); however, even this
allow only provides accessible viscosities of .about.10^Pa-s,
substantially higher than the viscosities needed to access most
thermoplastic forming techniques. (See, Schroers, J., et al.
Scripta Materialia, 2007, 57, 341-344.1
Accordingly, a need exists for a new family of inexpensive MGs that
can be incorporated into a thermoplastic processing
application.
SUMMARY OF THE INVENTION
The current invention is directed to a new class of amorphous
alloys that can be thermoplastically processed to make material
parts and articles, and methods of thermoplastically processing
such amorphous alloys.
The current invention is directed to BMG alloy compositions
comprising beryllium, at least one ETM, and at least one LTM, and
to methods of forming such BMG alloy compositions where at a
heating rate of 20 K/min the alloy has a .DELTA.T of at least 135 K
and a viscosity that falls below a value of less than about
10.sup.5 Pa-s. In one such an embodiment the composition is in
accordance with the equation:
(Zr.sub.xTi.sub.(1-x)).sub.a1ETM.sub.a2CU.sub.b1LTM.sub.b2Be.sub.c,
where x is an atomic fraction and a1, a2, b1, b2, and c are atomic
percentages, and where (a1+a2) falls within the range of 60 to 80%
and x is in the range of 0.05 to 0.95; and
In one embodiment, the invention is directed to quaternary BMG
compositions having a base composition of Be--Ti--Zr--Cu. In such
an embodiment up to 15% of the Ti or Zr can be substituted with
another element. In one such embodiment the additional element is
an early transition metal. Also, in such an embodiment, Cu can be
substituted with another late transition metal, such as Fe or
Co.
In another embodiment of the invention the ternary BMGs in
accordance with the current invention readily form an amorphous
phase upon cooling from the melt at a rate less than 10.sup.3
K/s.
The above-mentioned and other features of this invention and the
manner of obtaining and using them will become more apparent, and
will be best understood, by reference to the following description,
taken in conjunction with the accompanying drawings. The drawings
depict only typical embodiments of the invention and do not
therefore limit its scope.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the present invention
will be better understood by reference to the following detailed
description when considered in conjunction with the accompanying
drawings wherein:
FIG. 1a provides an overlay of a DSC scan and a viscosity curve of
the supercooled liquid region of a conventional amorphous
alloy;
FIG. 1b provides a data graph comparing the viscosities of a
conventional amorphous alloy and an exemplary alloy in accordance
with the current invention.
FIG. 2 provides a schematic TTT diagram showing two possible
thermoplastic processing routes (Johnson) versus the injection
molding processing route (TPF) described in the current
invention;
FIG. 3 provides a schematic diagram of a cavitated pore formed
during conventional die casting of a bulk part;
FIG. 4 provides a schematic TTT diagram showing the injection
molding processing route described in the current invention;
FIG. 5 provides a schematic diagram of an injection molding
apparatus in accordance with an exemplary embodiment of the current
invention;
FIG. 6 provides DSC scans of three typical bulk metallic glasses
with excellent glass forming ability and extremely high thermal
stability in accordance with the current invention;
FIG. 7 provides a comparison graph of the temperature dependence of
the equilibrium viscosity of several metallic glass forming
liquids;
FIG. 8 provides a comparison of TTT diagrams for several amorphous
alloys;
FIG. 9a to 9d provide photographs of a demonstration of the
thermoplastic processability of an exemplary metallic glass in
accordance with the current invention;
FIG. 10 provides photographs of exemplary injection molded parts in
accordance with one embodiment of the thermoplastic processing
methodology of the current invention;
FIG. 11 provides a comparison data graph of the rupture modulus of
a die cast piece versus a piece molded in accordance with the
injection molding process of the current invention; and
FIG. 12 provides a comparison data graph of the Weibull modulus of
a die cast piece versus a piece molded in accordance with the
injection molding process of the current invention.
DETAILED DESCRIPTION OF THE INVENTION
In general terms, the current invention is directed to producing a
new class of high strength, thermoplastically processable amorphous
alloys, which in the broadest terms are composed of Beryllium and
at least one ETM and LTM. The materials of the current invention
possess a unique combination of properties including, low density,
viscosities in the thermoplastic zone (at least one order of
magnitude lower than that of the commercialized Zr-based alloys and
lower also to the viscosity of Pd-based metallic glass and
approaching the viscosities attainable in polymer glasses), high
thermal stability (up to 165 K), low T.sub.g (about 300.degree.
C.), and good glass forming ability (critical casting thickness at
least 15 mm). As a result of these unique property combinations,
these alloys demonstrate good thermoplastic processability, and
combined with their excellent mechanical properties, these alloys
are appropriate for use in a number of applications, including
microelectromechanical systems, nano- and microtechnology, and
medical and optical applications. Moreover, the large supercooled
liquid region offered by these unique alloys in the current
invention enables Newtonian flow conditions at strain rates higher
than those of a conventional metallic glass with a smaller
supercooled liquid region. This capability can be utilized for more
efficient wire/fiber/plate/sheet drawing process.
DEFINITIONS
Early Transition Metal (ETM): For purposes of this invention, early
transition metals are defined as elements from Groups 3, 4, 5 and 6
of the periodic table, including the lanthanide and actinide
series. The previous IUPAC notation for these groups was IIIA, IVA,
VA and VIA.
Late Transition Metal (LTM): For purposes of this invention, late
transition metals are defined as elements from Groups 7, 8, 9, 10
and 11 of the periodic table. The previous IUPAC notation was VIIA,
VIIIA and IB.
Amorphous Alloys or Metallic Glasses (MGs): For purposes of this
invention, metallic glasses are defined as materials which are
formed by solidification of alloy melts by cooling the alloy to a
temperature below its glass transition temperature before
appreciable homogeneous nucleation and crystallization has
occurred.
Thermoplastic Processing (TPF): For the purposes of this invention,
thermoplastic processing/forming is defined as a processing
technique for forming metallic glasses in which the metallic glass
is held at a temperature in a thermoplastic zone, which is below
T.sub.nose (the temperature at which crystallization of the
amorphous alloy occurs on the shortest time scale, which means that
the resistance of crystallization is minimum) and above T.sub.g
(the glass transition temperature) during the shaping or molding
step, followed by a quenching step where the item is cooled to the
ambient temperature.
Extruding: For the purposes of this invention, extruding is defined
as either to force, press, or push out; or to shape (as metal or
plastic) by forcing through a die.
Injection molding: For the purposes of this invention, injection
molding is defined as a method of forming articles (as of plastic)
by heating the molding material to a temperature within the SCLR
until it can flow and injecting it into a mold.
Discussion of TPF Alloys
As discussed previously, one of the major limitations faced in
forming conventional amorphous alloys is the small processing
window available before crystallization, and the relatively high
viscosity of the material within that processing window. Forming
processes for these materials are further complicated by the
interrelation between the viscosity of the alloy and the
temperature at which the alloy crystallizes. To demonstrate this
FIG. 1a provides an overlay of a DSC scan and a viscosity curve for
one of the best conventional amorphous alloy showing how viscosity
drops in the supercooled liquid region until crystallization. As
shown, for these materials the lowest viscosities are accessible
close to Tx. (Note that in FIG. 1a the viscosity curve (inset) is
aligned with the temperature scale from the DSC curve.)
Unfortunately, in most amorphous alloys the supercooled region is
such that the viscosity remains too high for most thermoplastic
processing techniques at temperatures that allow the material to
retain its amorphous character. For example, typically metallic
glass viscosity .about.10^7 Pa-s whereas polymers are injection
molded at .about.10^3 Pa-s. In contrast, the viscosity of an
exemplary alloy of the current invention
(Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5) when measured at a
heating rate of 20 K/min is less than about 10^5 Pa-s, two orders
of magnitude lower than conventional amorphous materials, as shown
in FIG. 1b.
The strain rate sensitivity for the Vitreoy alloys has been
extensively studied (J. Lu, G. Ravichandran, W. L. Johnson, Acta
Materialia 2003, 51, 3429, the disclosure of which are incorporated
herein by reference). As is known from follow-up analysis of the
same experimental data, higher thermal stability of the supercooled
liquid can lead to a substantial increase of the strain rate limit
for Newtonian flow. Specifically, it has been shown that if the
supercooled liquid can remain stable at 135 K above the glass
transition temperature, at least 5 orders of magnitude increase in
the strain rate limit for Newtonian flow can be realized. (See, M.
D. Demetriou, and W. L Johnson, Scripta Materialia, 2005, 52, 833,
the disclosure of which are incorporated herein by reference.)
Newtonian flow conditions are necessary and important for
applications involving tensile loading, such as
wire/fiber/plate/sheet drawing. Non-Newtonian flow gives rise to
shear thinning that leads to necking and cessation of the process.
Therefore, a high strain rate capability while maintaining
Newtonian flow can enable a more efficient drawing process.
In general terms, the current invention is directed to producing
high strength, thermoplastically processable (TPF) amorphous alloys
which are composed of Beryllium and at least one ETM and at least
one LTM. An alloy optimal for TPF would have good glass forming
ability, low viscosity in the SCLR, a low processing temperature,
and a long processing time at that temperature before
crystallization. It has been found that Be-bearing Zr--Ti based
quaternary metallic glasses having compositions that fall within
the range of 60%<Zr+Ti<, 80% have Lower T.sub.g, and
increased SCLR in comparison with conventional bulk solidifying
amorphous alloys such as the Vitreloy alloys (Zr+Ti=55%).
More specifically, the amorphous alloys of the current invention
comprise Beryllium and at least one ETM and at least one LTM in
accordance with the formula:
(Zr.sub.xTi.sub.(1-x)).sub.a1ETM.sub.a2Cu.sub.b1LTM.sub.b2Be.sub.c,
where x is an atomic fraction and a1, a2, b1, b2, and c are atomic
percentages, and where (a1+a2) falls within the range of 60 to 80%,
x is in the range of 0.05 to 0.95. In addition, it is required that
Ni make up no more than a fractional amount of the overall alloy
composition, defined herein as less than 5% of the total alloy
composition.
In a preferred embodiment of the invention, the alloy formulation
may be expressed by the following formulation:
Zr.sub.aTi.sub.bCu.sub.cBe.sub.d, and falls within one of the
following sub-ranges where a+b+c+d equals 100%: where a+b>60%
with d>15% where a.apprxeq.b with d>15%; and where
a.apprxeq.5b with d>20%.
Although specific ranges of materials are provided above, it should
be understood that variations and modifications to the proposed
invention can exist with respect to the composition of amorphous
alloys. For example other elements, excluding ETMs and LTMs, can be
added to the alloys without significantly altering the base alloy
properties. Such materials may include, for example, Sn, B, Si, Al,
In, Ge, Ga, Pb, Bi, As and P. In addition, Cu can be substituted
with other LTMs such as, for example, Co and Fe, but in any event
the concentration of Ni in the alloy cannot exceed 5% of the total
alloy composition.
Regardless of the specific compositional substitutions made, the
two key distinguishing features of alloys made in accordance with
the above formulations are that when heated at a rate of 20 K/min
the alloys have supercooled liquid regions of at least 135 K, and
that at a heating rate of 20 K/min the alloys have processing
viscosities in the supercooled liquid region of less than around
10^5 Pa-s (unprecedentedly low for a metallic glass forming
system). Accordingly, the alloys of the current invention exhibit
"benchmark" characteristics for thermoplastic processing. Table 1
below, provides a listing of exemplary alloy formulations in
accordance with the above ranges along with thermal properties for
those alloys.
TABLE-US-00001 TABLE 1 Summary of BMG forming alloys investigated
in the current invention. Materials T.sub.g T.sub.x T.sub.l
.DELTA.T T.sub.rg Zr.sub.35Ti.sub.30Be.sub.30Cu.sub.5 574.9 725.3
1114.4 150.4 0.516 Zr.sub.35Ti.sub.30Be.sub.27.5Cu.sub.7.5 574.6
739.7 1070.7 165.1 0.537 Zr.sub.35Ti.sub.30Be.sub.26.75Cu.sub.8.25
578.2 737.2 1044.2 159 0.554
Zr.sub.54Ti.sub.11Be.sub.22.5Cu.sub.12.5 581 721 1035 140 0.561
Zr.sub.54Ti.sub.11Be.sub.17.5Cu.sub.17.5 584 722 1074 138 0.544
Zr.sub.51Ti.sub.9Be.sub.27.5Cu.sub.12.5 595 731 1042 136 0.571
Zr.sub.51Ti.sub.9Be.sub.25Cu.sub.15 592 730 1047 138 0.565
Zr.sub.40Ti.sub.25Be.sub.29Cu.sub.6 579.7 728.1 1113.1 148.4 0.521
Zr.sub.40Ti.sub.25Be.sub.27Cu.sub.8 579.4 737.5 1080.0 158.1 0.536
Zr.sub.40Ti.sub.25Be.sub.25Cu.sub.10 579.4 743.2 1046.9 163.8 0.553
Zr.sub.27.5Ti.sub.35Be.sub.29.5Cu.sub.8 590.9 728.6 1107.5 137.7
0.534 Zr.sub.32.5Ti.sub.30Be.sub.31.5Cu.sub.6 590.4 739.7
>1123.2 149.3 <0- .526
Zr.sub.32.5Ti.sub.30Be.sub.29.5Cu.sub.8 587.7 745.1 1092.9 157.4
0.538 Zr.sub.32.5Ti.sub.30Be.sub.27.5Cu.sub.10 587.8 747.4 1061.2
159.6 0.554 Zr.sub.37.5Ti.sub.25Be.sub.27.5Cu.sub.10 584.0 744.1
1080.2 160.1 0.541 Zr.sub.30Ti.sub.30Be.sub.32Cu.sub.8 591.2 736.0
1123.2 144.8 0.526 Zr.sub.30Ti.sub.30Be.sub.30Cu.sub.10 596.0 740.4
1046.0 144.4 0.570 Zr.sub.35Ti.sub.25Be.sub.32Cu.sub.8 596.5 735.4
1021.2 138.9 0.584 Zr.sub.35Ti.sub.25Be.sub.30Cu.sub.10 595.0 746.1
989.2 151.1 0.601 Zr.sub.35Ti.sub.25Be.sub.28Cu.sub.12 596.3 744.0
984.6 147.7 0.606 Zr.sub.40Ti.sub.20Be.sub.26.25Cu.sub.13.75 589.5
740.8 1114.7 151.3 0.529 Zr.sub.35Ti.sub.30Be.sub.33Co.sub.2 584.3
721.0 1097.3 136.7 0.532 Zr.sub.35Ti.sub.30Be.sub.31Co.sub.4 588.7
740.4 1075.1 151.7 0.548 Zr.sub.35Ti.sub.30Be.sub.29Co.sub.6 597.3
749.4 1110.5 152.1 0.538 Zr.sub.35Ti.sub.30Be.sub.33Fe.sub.2 586.0
722.8 1100.8 136.8 0.532 Zr.sub.35Ti.sub.30Be.sub.31Fe.sub.4 591.7
737.8 1073.7 146.1 0.551
Although the above discussion has focused on the formulation and
properties of the TPF alloy of the current invention, the invention
is also directed to novel techniques for forming and shaping such
materials. It should be understood as a starting point that the
formation of the alloy materials and the shaping of those materials
may either be intertwined or separate processes, and in the case
where separate processes are used to make the alloy material and
then form that material into a final product any suitable process
may be used to make the alloy starting material.
For example, in one common process nominal compositions are made
into ingots by melting the mixtures in an arc furnace under an
inert gas atmosphere. The alloy ingots are then cast into cavities
with different shapes within a conductive mold to render the
solidified product amorphous. In such an embodiment material parts
or articles can be made by thermoplastically processing the
amorphous sheets or amorphous starting materials with any suitable
thermoplastic processing technique as will be discussed in the
following section. It should be understood in reading the following
methods that any suitable method of making a feedstock of material
may be used, such as, for example, by a drop tower method, etc.
In one embodiment, the method of thermoplastically processing an
amorphous alloy may comprise a plastic molding process including
the steps of: providing a quantity of a metallic glass in an
amorphous state in the ambient temperature; heating said amorphous
alloy directly to an intermediate thermoplastic forming temperature
range above T.sub.g and below the T.sub.nose; stabilizing the
temperature of the amorphous alloy within the intermediate
thermoplastic forming temperature range; shaping the amorphous
alloy under a shaping pressure low enough to maintain the amorphous
alloy in a Newtonian viscous flow regime and within the
intermediate thermoplastic forming temperature for a period of time
sufficiently short to avoid crystallization of the amorphous alloy
to form a molded part; and cooling the molded part to ambient
temperature.
In another embodiment, the method of thermoplastically processing
an amorphous alloy may comprise a plastic casting process including
the steps of: providing a quantity of an amorphous alloy in a
molten state above the melting temperature of the amorphous alloy
(T.sub.m); cooling said molten amorphous alloy directly to an
intermediate thermoplastic forming temperature range above T.sub.g
and below the T.sub.nose; stabilizing the temperature of the
amorphous alloy within the intermediate thermoplastic forming
temperature range; shaping the amorphous alloy under a shaping
pressure low enough to maintain the amorphous alloy in a Newtonian
viscous flow regime and within the intermediate thermoplastic
forming temperature for a period of time sufficiently short to
avoid crystallization of the amorphous alloy to form a molded part;
and cooling the molded part to ambient temperature.
In still another embodiment, the method of thermoplastically
processing an amorphous alloy comprises an injection molding
process. For clarity, the steps of this process are overlaid on a
TTT diagram in FIG. 4. As shown, the process includes the steps of:
heating/cooling an amorphous feedstock to a temperature between the
glass transition temperature, T.sub.g, and the crystallization
temperature, T.sub.x (FIG. 4, Step 1); forcing the heated alloy
through a restrictive nozzle before entrance into a mold (FIG. 4,
Step 2); and cooling the molded part to an ambient temperature
(FIG. 4, Step 3).
The injection molding process requires several additional
components including a reservoir for the amorphous feedstock, a
method of heating the amorphous metallic feedstock, a method of
applying pressure to the material in the reservoir, a gate or
gating system, a mold and optionally a method of heating the mold.
One exemplary embodiment of such a system is diagrammed
schematically in FIG. 5. As shown, a reservoir (10) of molten alloy
is attached via a gate and nozzle (12) to a mold (14). A pressure,
in this case via a plunger mechanism (16) is then applied to the
alloy in the reservoir to inject it through the gate/nozzle into
the mold.
Although any suitable method of heating the amorphous feed stock
may be used with the injection molding process of the current
invention, some exemplary methods include, but are not limited to
an RF power supply and coil, a cartridge heater, and a furnace.
Likewise, suitable methods of applying pressure to the material in
the reservoir may include, but are not limited to, a piston, a
plunger, and a screw drive.
Although injection molding is generally considered more complicated
to perform than the conventional casting/molding processes
described above, there are several significant advantages that make
it attractive. For example, the most common method of obtaining
metallic glass parts is die casting where the molten alloy is
injected into a mold and then cooled below the glass transition
temperature sufficiently fast to avoid crystallization. However,
die casting requires the molten alloy to be rapidly quenched while
being molded in order to effectively bypass crystallization. This
processing route thus takes advantage of the thermodynamic
stability of the alloy at temperatures above the crystallization
nose (the point labeled as T.sub.n in FIG. 2), which provides the
temperature T.sub.n at which an alloy has the minimum time to
crystallization. However, using such a technique can introduce flow
defects into the sample such as micro-cavities, due to high
inertial forces in relation to the surface tension forces during
the injection of the low viscosity molten liquid. High inertial
forces in relation to surface tension forces give rise to a
Rayleigh-Taylor instability and consequent flow break-up, resulting
in void entrapment. Cavities are also found in the center of die
cast parts because parts are vitrified through contact with a mold
from the outside in, and cavities nucleate in the center due to the
built-up of negative pressure. This phenomenon is shown
schematically in FIG. 3. Other undesirable defects can also be
found in parts fabricated by the die casting method such as high
residual stress concentrations, arising due to a strong coupling
between high speed flow and rapid cooling. The flow and cooling
requirements of die casting also bound the dimensions of die cast
parts to no larger than that which can be cooled sufficiently fast
to avoid crystallization and no smaller than that which can be
quickly filled. Accordingly, parts with complex geometries, thin
sections, and high aspect ratios are difficult to obtain with die
casting.
As described above, plastic processing techniques where an
amorphous feedstock is heated to a temperature between T.sub.g and
T.sub.x and formed under pressure also exist. These methods
generally take advantage of the kinetic stability of the alloy at
temperatures below the crystallization nose (see, e.g., FIG. 5).
Plastic processing also takes advantage of lower processing
temperatures resulting in relatively lower oxidation rates These
methods include the forming of amorphous metal sheets (see, e.g.,
U.S. Pat. No. 6,027,586, the disclosure of which is incorporated
herein by reference), the compaction of amorphous powders (see,
e.g., U.S. Pat. No. 5,209,791, the disclosure of which is
incorporated herein by reference), the extrusion of amorphous
feedstock into a die (see, e.g., K. S. Lee, Y. W. Chang, 2005, the
disclosure of which is incorporated herein by reference), and the
imprinting of amorphous metal (see, e.g., Y. Saotome, et al., 2002,
the disclosure of which is incorporated herein by reference). While
most of these routes reduce the defects of the processed amorphous
part, each has other limitations. For example, forming amorphous
metal sheets limits the thickness of the final sample and the
available part geometries, powder compaction methods usually
produce parts having micro- or nano-dispersed porosity that often
results in inferior mechanical properties compared to
homogenously-solidifying parts, free extrusion, or extrusion into a
die only allows parts with simple geometries to be fabricated, and
imprinting methods enable very small features to be replicated, but
are incapable of producing bulk parts.
The present invention utilizes the ability of the TPF metallic
glasses of the current invention to flow homogeneously at
temperatures between T.sub.g and T.sub.x, to enable pressurized
injection of the alloy into a mold to produce a homogenous bulk
part with no size restrictions. Another method that utilizes the
flow capabilities of metallic glasses between T.sub.g and T.sub.x
has been invented by Johnson (See, U.S. Pat. No. 7,017,645, the
disclosure of which is incorporated herein by reference). That
method involves cooling the molten alloy from above the melting
point to a temperature between the crystallization nose and
T.sub.g, molding at this intermediate temperature, and cooling to
ambient temperature. Although this method has similar advantages to
the present invention in terms of achievable part geometries and
final porosity, Johnson's method requires bypassing the
crystallization nose during processing necessitating complicated
setups comprising hermetically sealed nozzles and diffusers.
Another disadvantage of Johnson's method is the smaller thermal
driving force available to quench at an intermediate temperature
before processing, as opposed to the current invention where an
amorphous feedstock can be quenched to room temperature and later
reheated for processing. As a result, Johnson's method necessitates
the use of alloys that exhibit high stability against
crystallization at T.sub.n whereas the method according to this
invention leaves open the possibility of using a broader range of
alloys.
The following examples are provided to demonstrate the improved
thermoplastic forming properties of the alloys of the instant
invention. Specifically tests were performed to investigate the
thermal, rheological, and crystallization
(Time-Temperature-Transformation (TTT)-diagrams) properties of the
inventive material. In summary these studies show that the alloys
of the current invention exhibit high yield strength, excellent
fracture toughness, and a relatively high Poisson's ratio. In
addition, simple micro-replication experiments carried out in open
air using relatively low applied pressures demonstrate superior
thermoplastic processability for engineering applications.
EXAMPLES
Example 1
Alloy Formation and Properties
Although any suitable alloy formation process may be used to form
the materials of the current invention, in the following examples
mixtures of elements of purity ranging from 99.9% to 99.99% were
alloyed by induction melting on a water cooled copper boat under a
Ti-gettered argon atmosphere. Typically 5 g ingots were prepared.
Each ingot was flipped over and re-melted at least three times in
order to obtain chemical homogeneity.
A Philips X'Pert Pro X-ray diffractometer and a Netzsch 404C
differential scanning calorimeter (DSC) (performed at a constant
heating rate 0.33 K/s) were utilized to confirm the amorphous
natures and to examine the isothermal behaviors in the SCLR of
these alloys.
The viscosity of Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5 as a
function of temperature in the SCLR was studied using a Perkin
Elmer TMA7 in the parallel plate geometry as described by Bakke,
Busch, and Johnson. (E. Bakke, R. Busch, W. L. Johnson, Applied
Physics Letters 1995, 67, 3260, the disclosures of which are
incorporated herein by reference.) The measurement was done with a
heating rate of 0.667 K/s, a force of 0.02 N, and an initial height
of 0.3 mm. The Viscosity and Temperature-Time-Transformation (TTT)
diagrams of Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5 at high
temperatures were measured in a high vacuum electrostatic levitator
(ESL). (See, S. Mukherjee, et al., Acta Materialia 2004, 52, 3689;
and S. Mukherjee, et al., Applied Physics Letters 2004, 84, 5010,
the disclosures of which are incorporated herein by reference.) For
the viscosity measurements, the resonant oscillation of the molten
drop was induced by an alternating current (AC) electric field
while holding the sample at a preset temperature. Viscosity was
calculated from the decay time constant of free oscillation that
followed the excitation pulse.
To determine the top half of the TTT curve, an electrostatically
levitated molten (laser melting) droplet (.about.3 mm diameter)
sample was cooled radioactively to a predetermined temperature, and
then held isothermally until crystallization. The temperature
fluctuations were within .+-.2 K during the isothermal treatment.
For temperatures below the nose of the TTT curve, data was obtained
by heating the alloy at 40 K/min in a graphite crucible to the
desired temperature and holding the sample isothermally until
crystallization.
Using the above techniques studies were performed on the physical
properties of alloys in the two "preferred" composition regions of
the current invention. As previously discussed, these "preferred"
regions include alloys that have compositions in accordance with
the following formula: Zr.sub.aTi.sub.bCu.sub.cBe.sub.d
(60%<a+b<80%), where in the first region a.apprxeq.b and
d>15%; and where in the second region a.apprxeq.5b and
d>20%
The differential scanning calorimetry (DSC) curves of three
representative alloys of the current invention are presented in
FIG. 6. The DSC scans (at a constant heating rate of 0.33 K/s) of
three typical metallic glasses with good glass forming ability and
high thermal stability against crystallization are presented. The
5-g samples were made in a Ti-gettered silver boat and were
generally found to freeze without any crystallization during
preparation resulting in a glassy ingot, which suggests that the
critical casting thickness of these alloys is at least 1.5 cm. The
downward arrows refer to the glass transition temperatures. As
shown, the alloys all exhibit a very large SCLR with a single sharp
crystallization peak at which the alloy undergoes massive
crystallization to a multiphase crystalline product.
The amorphous nature of all the samples studied in this work has
been confirmed by X-ray diffraction. A summary of thermal
properties of these alloys are listed in Table 2 below, and
compared with several earlier reported amorphous alloys.
TABLE-US-00002 TABLE 2 Thermal property comparison of various BMG
forming alloys. T.sub.g T.sub.x T.sub.l .DELTA.T Materials (K) (K)
(K) (K) T.sub.g/T.sub.l T.sub.TPF
Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5 575 740 1071 165 0.537
600-710 Zr.sub.41.2Ti.sub.13.8Ni.sub.10Cu.sub.12.5Be.sub.22.5 623
712 993 89 0.627- 640-690
Zr.sub.46.75Ti.sub.8.25Ni.sub.10Cu.sub.7.5Be.sub.27.5 625 738 1185
113 0.5- 27 650-710 Pd.sub.43Ni.sub.10Cu.sub.27P.sub.20 575 665 866
90 0.664 600-640 Pt.sub.60Ni.sub.15P.sub.25 488 550 804 60 0.596
510-530 Ce.sub.68Cu.sub.20Al.sub.10Nb.sub.2 341 422 643 81 0.530
360-400 Au.sub.49Ag.sub.5.5Pd.sub.2.3Cu.sub.26.9Si.sub.16.3 401 459
644 58 0.623 4- 20-440 Pt.sub.57.5Cu.sub.14.7Ni.sub.5.3P.sub.22.5
508 606 795 98 0.639 530-580 References: A. Peker, W. L. Johnson,
Applied Physics Letters 1993, 63, 2342; B. Zhang, et al., Physical
Review Letters 2005, 94, 205502; T. A. Waniuk, et al., Applied
Physics Letters 2001, 78, 1213; H. Kato, et al., Scripta Materialia
2006, 54, 2023; K. Shibata, et al., Progress of Theoretical Physics
Supplement 1997, 126, 75; and J. Schroers, et al., Applied Physics
Letters 2005, 87, 061912, the disclosures of each of which are
incorporated herein by reference.)
The variations of SCLR, .DELTA.T, (.DELTA.T=T.sub.x-T.sub.g, in
which T.sub.x is the onset temperature of the first crystallization
event) and reduced glass transition temperature T.sub.rg
(T.sub.rg=T.sub.g/T.sub.l, where T.sub.l is the liquidus
temperature) are calculated. In the alloys of the current
invention, Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5 exhibits the
lowest T.sub.g (575 K and about 50 K lower than that of Vitreloy 1
or Vitreloy 4) and the largest .DELTA.T. It was further found that
the .DELTA.T of the same glass can be maintained at .about.165 K by
addition of 0.5% Sn, providing the largest SCLR reported for any
known bulk metallic glass.
In FIG. 7, the temperature dependence of equilibrium Newtonian
viscosity of on exemplary alloy of the current invention
(Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5) and several other
metallic glass forming liquids are presented. In the figure, the
following symbols are used for the different materials:
Zr.sub.41.2Ti.sub.13.8Ni.sub.10Cu.sub.12.5Be.sub.22.5 (Vit1)
(.DELTA.); Zr.sub.46.25Ti.sub.8.25Cu.sub.7.5Ni.sub.10Be.sub.27.5
(Vit4) Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5 (.quadrature.);
Pd.sub.43Ni.sub.10Cu.sub.27P.sub.20 (x); and
Pt.sub.60Ni.sub.15P.sub.25 (.diamond.). The solid curve represents
a Vogel-Futcher-Tammann (VFT) fit to the viscosity data of
Zr.sub.35Ti.sub.30Cu.sub.7.5-Be.sub.27.5 in accordance with the
following equation:
.eta..eta..times..times..function..times..times..times.
##EQU00001## where .eta..sub.0, D*, and T.sub.0 are fitting
constants. T.sub.0 is the VFT temperature and
.eta..sub.0.apprxeq.10.sup.-5 Pa s. In the best fit, T.sub.0=422.6
K and D*=12.4 are found. The alloy in accordance with the current
invention shows a viscosity in the thermoplastic zone
(570.about.720 K) that is at least two orders of magnitude lower
than that of Vitreloy 1 or Vitreloy 4 at the same temperature and
is comparable to that of Pd-based metallic glass, but with a larger
.DELTA.T. For example, the equilibrium viscosity at 410.degree. C.
for Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5 is measured to be only
6*10.sup.4 Pas, similar to that of viscous polymer melts. (See, F.
W. Billmeyer, Textbook of Polymer Science, 1984, 305, the
disclosure of which is incorporated herein by reference.) As is
known from the processing of thermoplastics, the formability is
inversely proportional to viscosity. Accordingly, the low viscosity
in the SCLR of the TPF alloy of the current invention will result
in a low Newtonian flow stress and high formability. Therefore, the
present alloys are much more preferable for thermoplastic
processing than the traditional Vitreloy 1 series.
In FIG. 8, we present the measured TTT curve for
Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5 and other Vitreloy series
alloys. (T. Waniuk, et al., Physical Review B 2003, 67, 184203, the
disclosure of which is incorporated by reference.) In the figure,
the following symbols are used for the different materials:
Zr.sub.41.2Ti.sub.13.8Ni.sub.10Cu.sub.12.5Be.sub.22.5 (Vit1) (x);
Zr.sub.46.25Ti.sub.8.25Cu.sub.7.5Ni.sub.10Be.sub.27.5 (Vit4) (*);
Zr.sub.44Ti.sub.11Cu.sub.10Ni.sub.10-Be.sub.25 (Vit1b) (+)) and the
selected Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5 alloy
(.quadrature. and .DELTA.). The data are measured by electrostatic
levitation (.quadrature.) and by processing in graphite crucibles
(other than .quadrature.) after heating from the amorphous state.
The processing window can be identified from this TTT diagram.
Specifically, the TTT curve indicates a nose shape, with the
minimum crystallization time of .about.3-10 s occurring somewhere
between 700 K and 950 K. At 680 K, where the equilibrium viscosity
is on the order of 10.sup.4 Pa s, a 600-s thermoplastic processing
window is indicated. Based on the curves it can be estimated that
the exemplary TPF alloy should have a processing time of about 2
minutes at around 700 K without risking crystallization.
To demonstrate the good thermoplastic processability of the
exemplary TPF alloy (Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5)
glassy alloy, thermoplastic imprinting experiments were performed
as shown in FIGS. 9a to 9d. The thermoplastic processing was done
on a Tetrahedron hot press machine in the air at a pressure of 25
MPa with a processing time of 45 s, followed by a water-quenching
step. FIG. 9 shows the microformed impression of a United States
dime coin (FIG. 9b) made on the surface of metallic glass wafers at
.about.370.degree. C. (FIG. 9a) indicating the excellent
imprintability and viscous deformability of the material. In
addition, minimal oxidation was observed after the processing which
is consistent with the strong oxidation resistance of Be-bearing
amorphous alloys. Finally, the final parts remain fully amorphous
as verified by X-ray diffraction. It is further found from the
Rockwell hardness tests that no damage to the mechanical properties
of the alloy was caused by the thermoplastic processing.
Before the TPF was carried out, diamond-shape micro-indentation
patterns (.about.100 .mu.m) were deliberately imprinted into the
wafer in the top flame of the dime using a Vickers hardness tester
(FIG. 9c). FIG. 9d presents the successfully replicated diamond
pattern in the final part. Even the scratches (on the level of
several .mu.m) on the original dime are clearly reproduced. The
results indicate a substantial advance in thermoplastic processing
of amorphous metals.
Accordingly, the metallic glass forming alloys of the current
invention have a combination of properties ideally suited for TPF
processes, such as extraordinarily low viscosity in the
thermoplastic zone, exceptional thermal stability, very low
T.sub.g, and excellent GFA. These alloys have also demonstrated
strong thermoplastic processability and excellent mechanical
properties providing for the possibility of broadening the
engineering applications of amorphous metals generally.
Example 2
Injection Molding Application
As discussed above, the current invention is also directed to novel
methods of forming the TPF alloys of the current invention. In FIG.
10 photographs are provided of parts made in accordance with the
novel injection molding process disclosed herein next to a polymer
part created from the same mold. (From top to bottom: Top Metallic
glass Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5 injected at 400 C
with 10000 PSI, 2.sup.nd same glass injected at 380 C with 45000
PSI, 3.sup.rd same glass injected at 420 C with 45000 PSI, and
4.sup.th Polymer part injected at 220 C with 5000 PSI, all parts
are as cast.) Slight polishing after molding with 320 grit paper
removes any oxide layer.
Due to the viscous nature of metallic glasses in the region between
T.sub.g and T.sub.x, the sprue and nozzle commonly used for plastic
injection molding were replaced by a thin washer that acted as a
nozzle. The TPF alloy Zr.sub.35Ti.sub.30CU.sub.7.5Be.sub.27.5, in
accordance with the current invention was used as the amorphous
feedstock to demonstrate the injection molding process because it
provides the largest supercooled liquid region (SCLR)
(T.sub.x-T.sub.g=165 C) of any alloy to date and also the lowest
attainable viscosity in the SCLR (.about.10.sup.4 Pa-s) of any
known metallic glass. The flashing is 0.1 mm thick and 2.5 mm wide,
and was formed mainly due to the lack of adequate clamping force
during the process. In this exemplary embodiment both sides of the
mold were not filled due to insufficient space in the reservoir for
enough material. These final parts demonstrate that a true
injection molding process can be used with the TPF alloy materials
of the current invention opening up new applications for these
alloys in industry.
FIG. 11 shows three point beam bending tests of 2 mm.times.2
mm.times.20 mm injection molded specimens and die cast specimens of
Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5. The average value of the
modulus of rupture is nearly equal for both processing methods, but
the standard deviation of the modulus of rupture for the cast
samples (2.879+/-0.240 GPa) is 3.7 times larger than that of the
injection molded specimens 12.923+/-0.065 GPa). FIG. 12 provides a
fit of the modulus of rupture data to obtain the Weibull modulus
for the injection molded specimens and die cast specimens of
Zr.sub.35Ti.sub.30Cu.sub.7.5Be.sub.27.5. Weibull modulus is
basically a measure of the reproducibility of parts. Weibull
statistics assume that failure initiates from defects in the
sample. Accordingly, samples with low Weibull modulus have high
numbers of defects per unit volume. In the current test the
injection molded parts made have Weibull modulus value of
m.sub.IM=41.9, while the die cast parts have a Weibull modulus of
m.sub.DC=9.74. As a comparison, high quality engineering ceramics
have Weibull modulus values of 1-10, while most metals have Weibull
modulus numbers greater than 100.
Both the modulus of rupture test and the Weibull modulus fit are
evidence of the improved mechanical properties and reproducibility
of fabricated part strengths due to the nearly defect free
structures found in parts produced by the injection molding
technique of the current invention.
SUMMARY
In summary, a new class of high strength, thermoplastically
processable amorphous alloys having low density, viscosities in the
thermoplastic zone at least two orders of magnitude lower than that
of the commercialized Zr-based alloys and similar to the viscosity
of Pd-based metallic glass and polymer glasses, unusually high
thermal stability, low T.sub.g, and excellent glass forming ability
(critical casting thickness .about.15 mm) have been discovered. In
addition, an injection molding technique has been developed to
allow processors to take full advantage of the unique properties of
these materials The technological potential of this class of glassy
alloys and the injection molding technique is very promising in a
wide-variety of applications including, for example, aerospace and
astrospace components (Ribs, spars, airframes, space structures),
defense (Armor plating, weapons), sporting goods (tennis rackets,
baseball bats, golf clubs), structural components (frames, casings,
hinges), automotive components, foam structures, nano- and
microtechnology, medical and optical applications, data storage,
and microelectromechanical systems.
Finally, it should be understood that while preferred embodiments
of the foregoing invention have been set forth for purposes of
illustration, the foregoing description should not be deemed a
limitation of the invention herein. Accordingly, various
modifications, adaptations and alternatives may occur to one
skilled in the art without departing from the spirit and scope of
the present invention.
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